Human embryo fibroblasts (IMR90 and HLM18) and cells (fibroblasts) grown from young and old donors in culture undergo a period of rapid proliferation which is followed by a decline in the rate of cell proliferation and the eventual degeneraton of the culture. Hayflick and Moorhead (1961) have suggested that the basis for this phenomenon is genetically intrinsic to the cell and may represent aging at the cellular level. The external cell surface appears to possess the functions of a sensory and communicating organelle for a single cell and considerable recent evidence indicates that the cell may use these functions to its advantage in the regulation of cell division and cell development, depending upon signals dictated through the extracellular environment. Whether gene products expressed at the cell surface differentiate with age and whether they play a role in the senescent behavior of cells aging in culture is presently unclear and is the subject of this program. Specific objectives include: 1. determination of whether cycle-dependent variation in membrane organization exists in early and late passage human embryo fibroblasts and cells from old and young donors, utilizing techniques of freeze-cleaving and electron microscopy; 2. further structural characterization of communicating (gap) junctions in each culture group; 3. a comparative analysis of the potential for metabolic coupling in all cell groups, utilizing autoradiography and mutant cell lines capable of incorporating selected precursors only when coupled to previously labeled cells; 4. further analysis of binding patterns of plant lectins (specifically concanavalin A, wheat germ agglutinin and ricinus communis agglutinin) on exterior surfaces of cells in each group by techniques of flow microsfluorometry and scanning and transmission electron microscopy; and 5. further investigation of whether reception of extracellular signals (e.g. norepinephrine) varies either with age- or experimentally-induced alterations in cell surface glycosaminoglycan.